1. Field of the Invention
The present invention relates to an optical device as well as a projector unit and a rear projector unit using the same.
2. Description of the Related Art
A projector system is drawing attention lately as a large screen display unit and a CRT projector system using a small, high definition and luminous CRT, a liquid crystal projector system using a liquid crystal panel, a DMD projector system using DMDs (Digital Micro-mirror Device) and others are manufactured. Among them, the DMD is one of light bulbs for the projector system which is being expected most because it is congenial to displaying computer information and to digital TV broadcasting in principle because it is a digital driving element.
The structure of the DMD will be explained at first. The DMD is fabricated by arraying a large number of micro-mirrors of 16 .mu.m.times.16 .mu.m for example on a silicon substrate so as to control reflecting directions of incident light per each mirror by electrically controlling them. The respective micro-mirrors correspond to pixels and image information may be displayed by projecting the light on screen via a projector lens. FIG. 1 is a perspective view showing the structure of two pixels of DMD elements. The figure shows micro-mirrors 510 and 511 which are inclined from the normal line of an element 600 by +10 degrees or -10 degrees, respectively.
Aluminum is evaporated on the surface of the micro-mirrors 510 and 511 so that the micro-mirrors operate as square mirrors for example having high reflectivity. The direction in which light is reflected may be switched by the tilting angle of the micro-mirrors and the light intensity from DMD is controlled by pulse width modulating (PWM) the micro mirrors. Such micro-mirrors are arrayed two-dimensionally in accordance to a predetermined format of 480.times.640 or of 600.times.800 for example to construct as a light bulb. The DMD is detailed in Larry J. Hornbeck, "Digital Light Processing for High-Brightness, High-Resolution Applications", SPIE Vol. 3013, pps. 27-40 and others, so that a further detailed explanation thereof will be omitted here.
Next, the principle of an optical system of a video projector using the DMD as a light bulb will be explained with reference to FIG. 2. FIG. 2 is a conceptual diagram for explaining the principle of display of the DMD by means of a single plate DMD projector unit which comprises an illuminant 150 such as a metal halide lamp, a DMD 601, a light absorber 602 for absorbing unnecessary light and a projector lens 80. In the figure, micro-mirrors 512 and 513 which are turned (tilted) by +10 degrees and a micro-mirror 514 which is turned (tilted) by -10 degrees in the DMD 601 are conceptually enlarged. In FIG. 2, light emitted from the illuminant 150 enters the DMD elements from the direction tilted by +20 degrees from the normal direction of the DMD 601. The light reflected by the micro-mirror 514 which is turned by -10 degrees deviates from the projector lens 80 and is absorbed by the light absorber 602, thus becoming a pixel of a black point on a screen not shown. Meanwhile, the light reflected by the micro-mirrors 512 and 513 which are turned (tilted) by +10 degrees are condensed by the projector lens 80, thus becoming pixels of bright points on the screen. Thus, the micro-mirror image on the DMD 601 is enlarged and projected on the screen as an image. It is also possible to display a color image by a single plate projector unit in which a rotary color filter is disposed within the optical path or by a three plate type projector unit which separates light into three primary colors of RGB and modulates per each color by arraying a dichroic prism and a dichroic filter.
By the way, as it is apparent from the operating principle of the DMD described above, the most characteristic condition in illuminating the DMD is that the illuminating light must be inputted with a predetermined angle from the normal direction of the plane-to-be-illuminated.
FIG. 3 is a schematic diagram showing the structure of a prior art color image display unit for displaying color images by using the DMD. It has been disclosed in JP-A-10-039240 for example. In FIG. 3, the unit comprises a DMD 603, micro-mirrors 515 and 516 composing the DMD, a parallel white color illuminant 151 emitting parallel white light, an optical thin film 400 causing transmission light of specific wavelength corresponding to an incident angle, an imaging lens 800 and a screen 900.
What should be noticed most here is the disposition of the parallel white color illuminant 151 with respect to the DMD 603. That is, the parallel white illuminant 151 is disposed in the direction inclined by a predetermined angle .alpha.1 with respect to the DMD 603 so as to input the parallel white light to the respective mirrors arrayed two dimensionally on the DMD 603. The white light incident on the DMD 603 is reflected by the mirrors 515 and 516 whose tilting angle .theta.1 is controlled and is then guided to the screen 900 by the imaging lens 800. While the optical thin film 400 is inserted to display color images, an explanation of its operation will be omitted here. The oblique illumination described above is required not only in the single plate projector unit of this example but also in a DMD projector unit of the type of a plurality of plates in common.
It is apparent that when the plane-to-be-illuminated is illuminated by concentric illuminating light fluxes, i.e., fluxes having a distribution of intensity rotationally symmetric about an optical axis from the direction tilted from its normal line, a distribution of the illuminated light is not concentric on the illuminated plane.
FIG. 4 is a diagram schematically showing the DMD 604 illuminated by the illuminating light fluxes having the concentric distribution of intensity and seen from the normal direction. Curves within the figure conceptually show equi-intensity lines by connecting points of the equal light intensity. Here, the flux of the illuminating light is inputted to the DMD 604 from the direction tilted by 45 degrees from one edge as indicated by an arrow in the figure and so that the optical axis of the illuminating light flux is tilted by 20 degrees from the normal line of the surface of the DMD. Accordingly, there arises a problem that the distribution of light intensity on the DMD surface is not concentric about the center point of the DMD 604 as shown in the figure and uneven illumination asymmetric in the up and down and right and left directions which is very inappropriate as a projector unit occurs on a screen. It is noted that the distribution of intensity in the present specification means the distribution of intensity of light within a plane vertical to the optical axis of the light flux.
The uneven illumination maybe relatively readily reduced by digital signal processing in case of the DMD projector unit. That is, it may be achieved by standardizing the intensity of projecting light per pixel by decreasing the gradation within the screen more than the original gradation for example so that the distribution of illuminance is uniformed on the whole screen based on a pixel to which the illuminating light of the least intensity enters. However, the improvement of the image quality by means of such light reducing process is not desirable from the aspect of the utility factor of the light. Because the DMD is a reflecting type light bulb, it has a merit that it is relatively strong against heat as compared to a transmission type light bulb and allows a high output illuminant such as a xenon lamp and a metal halide lamp of several hundreds W to 2 or 3 kW classes to be used. However, the rate of the light of the illuminant reaching to the screen in the end, i.e., the utility factor of the light, has stayed around several % with respect to the original output of the light of the illuminant similarly to the projector units using the other light bulbs. Accordingly, it has been important to reduce the uneven illumination caused by the optical system also in order to improve the utility factor of the light in each structural element.
Then, an illuminating optical system using a mixing rod for example has been employed in the prior art projector unit in order to enhance the uniformity of illumination of the light bulb.
FIG. 5 is a conceptual diagram of the illuminating optical system using the mixing rod having rectangular input and output end faces. In the figure, the system comprises an illuminant 152, a reflecting mirror 200, a mixing rod 300, a lens 401, and a light bulb 605. Light emitted from the illuminant 152 is condensed by the reflecting mirror 200 and is inputted to the mixing rod 300 disposed so that its input end face is positioned in the vicinity of the condensing point.
The light is mixed while propagating within the mixing rod 300 by repeating total reflection by several times at the interface of the glass and air. This mixing reduces the uneven brightness of the illuminating flux peculiar to the illuminant 152 and to the reflecting mirror 200 and allows a highly uniform diverging flux whose section is rectangular to be obtained at the output end face of the mixing rod 300. Accordingly, the uniformity of illumination of the light bulb 605 may be improved efficiently by forming the image of the output end face of the mixing rod in the vicinity of the light bulb 605 by the following lens 401. Thus, the method by means of the mixing rod 300 is characterized in that the illuminating flux having the desirable uniformity and shape of flux may be set totally independently from the following illuminating optical system. See U.S. Pat. No. 5,634,704 about the detail of the illuminating system using such a mixing rod. It is noted that the shape of flux in the present specification refers to the sectional shape of the flux on a plane vertical to the optical axis of the flux.
The uniform illumination method by means of the mixing rod as described above allows the maximum effect to be obtained when the light bulb, i.e., the plane-to-be-illuminated, is disposed vertically to the optical axis of the illuminating light. However, it has caused a problem that the angle of the image of the output end face of the mixing rod does not coincide with that of the surface of the DMD and uneven illumination occurs unavoidably after all even when the illuminating flux is inputted from a predetermined direction with respect to the DMD by disposing a plane mirror within the illuminating light path.
In a multi-projector system in which a large screen is constructed by arraying a plurality of projector units (or rear-projector units) horizontally and vertically, it is strictly required not only to improve the light utility factor but also to make borders of screens inconspicuous by minimizing the difference of brightness and color among the respective screens or the difference of brightness and color in the vicinity of the borders of the arrayed projected screens in particular. Accordingly, there has been a problem that the uniformity of brightness and color must be enhanced in maximum and the uneven illumination must be eliminated in the projected screen of each projector unit (or the rear-projector unit).